This is so that the format with a text_tyle will compile even if
header-only mode isn't enabled. Addresses #1063.

percentage.

This helps with compatibility with Python's format strings.

* Fix order of initializers

* Remove unused parameter

* Disable warning "-Wswitch-enum"

* Clarify usage of fmt::arg

Document that fmt::arg takes a non-owning
reference, even if that reference is to
a temporary. As such, users should make sure
the lifetime of the reference lasts as long
as the named argument.

* Clean up language

Remove mentions of `std::reference_wrapper` and rvalues
in favor of more common terminology like dangling references.

Patch by Ievgen Liubymkin (jackyf).

TravisCI
- replaced from manual Gradle download to install with 'sdkman'
- changed build name

Gradle
- updated build tool version

Since C++17, using template specialization 'std::allocator<void>' in any shape or form (even just mentioning it) is deprecated! A simple workaround is replacing 'void' by another (preferably empty) type, e.g. 'fmt::monostate'.

Found by Clang 9 in Visual Studio.

Name hiding.
Signed-off-by: Daniela Engert <dani@ngrt.de>

C++17 deprecated 'std::result_of' in favour of 'std::invoke_result' and will ban it outright in C++20. Therefore
- implement 'internal::result_of' in terms of 'std::invoke_result' when compiling C++17 mode.
- implement 'internal::result_of' in terms of 'std::result_of' when compiling in modes C++11 or C++14.
Signed-off-by: Daniela Engert <dani@ngrt.de>